Hacking Silence: Attacks via ultrasound and inaudible commands

In recent years, a more “silent” category of attacks has emerged that exploits something almost invisible to humans: the sound we cannot hear. The so-called “silent hacking” is based on the use of ultrasound and inaudible commands, i.e. voice commands that are outside the human hearing range but can be recognized by devices.

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Humans typically hear frequencies between 20 Hz and 20 kHz. Beyond this range, sound becomes practically invisible to human perception. However, modern devices, such as smartphones, smart speakers and digital assistants, use microphones that can record a wider range of frequencies. This creates a window of opportunity for attacks that transmit commands in ultrasonic form, which humans cannot perceive but the devices carry out normally.

A typical example is attacks via inaudible voice commands. In these cases, an attacker can embed a hidden command within an audio signal or even music. The user hears something completely innocent, while their device “hears” a different command, such as opening a website, sending a message, or activating a function. This phenomenon highlights a fundamental problem: machines do not perceive the world like humans and can be deceived in ways that we cannot.

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These attacks are not limited to digital assistants. Research has shown that ultrasonic signals can interfere with sensors such as gyroscopes and accelerometers, leading to incorrect measurements. In more critical systems, this could have serious consequences, especially in environments such as the automotive or aerospace industries, where sensors play a crucial role.

The most worrying thing is that these attacks are difficult to detect. They leave no visible traces, do not rely on traditional software vulnerabilities, and can be executed remotely, even through everyday devices like speakers or televisions. Their “silence” is precisely their advantage, as the user has no indication that anything is happening.

Addressing this threat requires new approaches to security. Device manufacturers are beginning to incorporate filters that limit the frequencies that microphones can process, as well as mechanisms that require user confirmation for critical commands. At the same time, research into the detection of anomalous sound patterns is ongoing.

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“Hacking silence” reveals a new battlefield in cybersecurity, where the attack is not through screens or keyboards, but through natural phenomena that escape human senses. As our devices become smarter and more connected, such attacks are expected to become more sophisticated. The key lesson is that security is not only about what we see or hear, but also what remains hidden beyond the limits of our perception.